Servicing of motor vehicles often requires that the mechanic perform his duties on the exposed underside of the vehicle. Service stations use hoists to lift vehicles for maintainance and repair. Vertical hoists are used to raise vehicles from underneath to selected positions above ground level. Such hoists generally are comprised of one or two hydraulic cylinder rams which move along their central axis longitudinally up and down to raise and lower the vehicle. Positioned on top of the hydraulic ram or rams is a support platform mechanism for engaging the underside of the motor vehicle and lifting the vehicle above ground.
The single ram hoist may appear generally in at least two embodiments. The first version is a single ram positioned to be placed below the center of the motor vehicle, with at least four pivotable spider arms with pads for engaging the outer frame or axles of the vehicle. An alternative single ram hoist is one which also is positioned central under the vehicle, having a secured pair of running tracks lying in a rigid horizontal frame which the vehicle drives along and which lifts the vehicle by its tires or frame. The advantages of the first embodiment are that one can adjust the location of the spider arms to engage the frame of vehicles designed for frame lift and engage axles or suspension mounts of those vehicles with unibody construction designed for lifting off the frame. The running track ram lift supports the automobile or truck as it usually supports itself, by the tires, axles and suspension system of the vehicle. A disadvantage of both systems is that distribution of the load of the vehicle is unevenly translated to the center pole hydraulic ram, since, in front engine cars, for example, the bulk of the weight is in front. This could cause a build-up in compressive loads in parts of the vehicle which would be structurally damaging. Additionally, certain types of servicing is best provided when the tires swing free. This is difficult to accomplish with a running track support mechanism.
Two ram hydraulic lifts positioned center front and rear below the respective front and rear tire axis laterally disposed across the width of the vehicle serve to distribute load by dividing the compressive force front and rear. One embodiment of a two ram hoist is a single frame running tracks which is secured atop the front and rear rams and functions like the single ram with running track frame construction. Although force distribution is improved over the single ram construction, the running track may not be adjusted to engage a variety of vehicles and relies on the tires.
An alternative embodiment of the twin ram hoist conventionally has two separate support platforms, one atop each ram, front and rear, having a fixed axle cradle for lifting the vehicle by its front and rear axle. The axle cradles allow the vehicle to be supported while the tires swing free and to distribute the weight of the vehicle independently front and rear. This embodiment also avoids lifting a vehicle by its outer frame preventing vehicular structural damage.
Beginning in the l970's, more vehicles have been designed with four wheel independent suspension systems. Independent suspension has resulted in designs which remove the rear axle in some systems. Four wheel independent suspension have shock tower control arms on the rear system which may trail or which lead off center from an axis running through the center of the rear tires across the vehicle.
The twin ram fixed axle cradle lift design of the conventional art presupposed the existence of front and rear axles. For servicing vehicles of four wheel independent suspension system design, a newer design is required which comprehends the fact that the rear end support mechanism has no rear axle to engage.
One solution to this problem is the conventional design of the "Weaver" adaptor tire lift. The rear support mechanism is a platform upon which the rear tires ride. In this manner, the rear end of a vehicle, lacking a rear axle, may be hoisted. Although this design for a rear support platform so1ves the problem of rear vehicular engagement where the vehicle has no rear axle, lifting by rear tires may limit servicing versatility since the tires do not swing free. Furthermore, if the front axle is engaged by a front axle cradle mount, while the rear wheels are supported by the rear "Weaver" adaptor support platform, the front ram would be raised higher than the rear ram in order for the vehicular underside to be supported parallel to the ground.
Furthermore, there may be an uneven distribution of load where the vehicular front end is supported by the front axle while the rear end is supported by its tires.
In order to properly service 4-wheel independent suspension vehicles, there is needed a twin ram vertical hoist support mechanism which will lift the rear underside of a variety of vehicles, some having rear axles and others not. The support mechanism must provide a sturdy engagement of the vehicle while allowing the tires to swing free The support mechanism must suspend the vehicle in the air parallel to the ground so that the front and rear hydraulic rams may be equally extended for maximum service.